Photos not importing all NEF files

I'm running Photos on Catalina, and when I import a folder of photos it imports the JPGs but mostly skips the raw NEF files. Of the 40 or so in the directories, it imported 10. If I select the skipped NEF files manually it will import them, but if I try the folder again it tells me it has already imported all the photos.

I've seen other questions where it was described that Photos "stacks" the RAW & JPG, such that only the JPG is shown. But that doesn't appear to be the case here, as I can create a smart album using filename NEF and I see only the 10 that were imported. Further, for the photos where I have both JPG and NEF, the Image->Use RAW as Original option is greyed out. So it doesn't appear to be stacked.

When I import the NEF directly it shows up in Photos as a separate photo. So it seems that I have to select all my NEFs manually rather than leaning on Photos to find them in my directories. Is that expected?

Note, I am not backing up my photos to iCloud, which may matter since I know Photos tries to optimize local disk space and store RAW files in the cloud. My camera is a Nikon D7500, which is supported by Catalina.